The present invention relates to a television receiver, especially to a television receiver which is mounted in a mobile unit and can provide high quality video images. A typical example of television receivers mounted in traveling bodies or mobile units (e.g., those mounted in automobiles) is described in Japanese Patent Application Laid-open No. 56-10780. This example maintains an optimum receiving state when an automobile moves to different reception environments with the following components and in the following manner: Four diversity antennas are mounted on the automobile, and there is a switching circuit which connects an output from one of these four antennas to an antenna input circuit of a television receiver mounted on the automobile. The detection output levels of the antennas are compared by time division switching of the switching circuit with a sampling switching control signal generated during a vertical blanking period of a television video signal. The antenna with the highest level is connected to the antenna input circuit of the television receiver. A control circuit maintains this state until the next vertical blanking period.
This conventional technology can supply a high quality video image when in a relatively good receiving environment, because reception is achieved by continuously switching the antennas so that the input waves with the strongest electric field intensity is selected. When the mobile unit is located in an area with many buildings, such as in the central part of cities, and ghosts are frequently generated, this method is not able to fully realize its potential because of the following reasons.
When ghosts exist, the conditions of ghosts generally changes when the automobile moves. Therefore, when the ghost signal is especially strong, the DU ratio (desired signal to undesired signal ratio) is inverted so that the ghost signal becomes the original main signal, and the main signal is considered the ghost signal. In other words, a time difference will occur in the main signal before and after the DU ratio is inverted. A sync-generator of the receiver will follow the currently input signal to synchronize with it, but it will momentarily be unable to follow the signal, then operate according to a delay time constant. During this period, a time difference will occur between the synchronizing signal in the video signal and that generated by sync-generator, so that video image will be distorted. Furthermore, if the DU ratio is frequently inverted, this video distortion will also become more frequent, so that the quality of reception will deteriorate significantly.
When opposite phase ghosts are being generated and the level of opposite phase ghosts rises, the white peak location of the video signal of the opposite phase ghost will become lower than the pedestal level of the main signal, so that sync-separation output will occur. In other words, because a synchronizing signal other than the original horizontal synchronizing signal is generated, the synchronization is easily distorted. If the level of the opposite phase ghost becomes even higher, the level of the white peak location of the opposite phase ghost will become lower than the level of the horizontal synchronizing signal, so that synchronization in the sync-generator will be completely disrupted.
When fading causes drastic fluctuations in the amplitude of the synchronizing signal, it becomes difficult to separate the synchronizing signal from the video signal, so that synchronization becomes distorted.